The polarization demultiplexing technique may double the spectrum utilization rate or capacity of an optical communication system, by optically modulating different information for two channels of carriers under orthogonal polarization state in space. On the other hand, the coherent detection technique can transit amplitudes and phase information of two optical signals in the orthogonal polarization direction to the baseband, so that all polarization information can be reserved, which makes it possible to perform polarization demultiplexing by using the signal processing technique in the digital baseband field.
Different from a mobile communication, an optical network shall provide a continuous and transparent transmission function for the upper service, and this requires the signal processing function of the optical receiver to be unrelated to the transmitted information as far as possible, or any signal processing is a non-data-aided processing. Presently, two non-data-aided polarization demultiplexing algorithms are commonly used, i.e., constant modulus algorithm and decision directed algorithm.
Andreas Leven etc. implement the constant modulus algorithm via FPGA, and carry out a real-time polarization demultiplexing test of 10 Gbit/s successfully (“A real-time CMA-based 10 Gb/s polarization demultiplexing coherent receiver implemented in an FPGA”, OFC2008, Paper OTuO2). The principle of the constant modulus algorithm (CMA) is to make the module value of the output signal approach the reference value as much as possible, by adjusting a group of filter coefficients. This process needs not to know the transmitted information, thus CMA is a blind algorithm. But CMA requires the transmitted signal to have constant power characteristics (e.g., PSK signal), and its application is limited.
The major problem of CMA is irregularity, i.e., two channels of output signals converge to the same signal source. The cause of CMA irregularity is that the cost function of CMA per se is a multi-extreme function, the filter coefficient leading to irregularity is still the locally optimal solution, and during the iterative process, CMA will easily converge to the locally optimal point. This problem is particularly obvious when a polarization dependent loss (PDL) exists in the system.
T. Pfau etc. implement the decision directed algorithm via FPGA, and successfully carry out a real-time polarization demultiplexing test of 2.8 Gbit/s (“Polarization-multiplexed 2.8 Gbit/s synchronous QPSK transmission with real-time digital polarization tracking”, ECOC2007, Paper 8.3.3). The principle of decision direction (DD) is to optimize the filter coefficient by replacing the training sequence in the conventional LMS method with the decision output data. The major problem of the method is the converging speed, and if the channel condition is poor, a convergence even cannot be implemented. In addition, the method further needs to construct the reference signal with the phase error estimated by a phase recovery module, thus is relatively complex.
In addition, CMA and DD methods can be combined. S. J. Savory captured signals with CMA in the off-line test, and when the signals are primarily divided, the DD algorithm is started to track the signals (“Transmission of 42.8 Gbit/s polarization multiplexed NRZ-QPSK over 6400 km of standard fiber with no optical dispersion compensation”, OFC2007, OTuA1). Although the problem of converging speed in DD is avoided, the problem of irregularity in CMA cannot be avoided.